In the field of impact printing, one of the most common types of printers has been the wire printer for printing characters in a dot matrix manner. As is well-known, the impact operation depends upon the movement of the wires which are typically moved by means of an electromechanical system which is believed to enable a precise control of the impact members.
One manner of wire matrix printing utilizes a single wire carried in side-to-side motion and driven into impact with paper or like record media upon energization of an electromagnetic solenoid. Another manner of wire matrix printing utilizes a print head having a plurality of wires disposed in alignment for printing characters as the print head is moved in side-to-side motion and as the wires are driven into impact with the paper upon energization of associated electromagnetic solenoids.
It is desired to increase the print quality of wire matrix printers and accordingly to eliminate or at least to substantially reduce certain problems in these printers. These problems or trouble areas can include determining whether the ink ribbon provides sufficient ink during the printing operation, whether the actual dot positions correspond with the desired positions and whether the print wires are operating in proper manner during the course of printing.
It is therefore proposed to observe the printing operation by way of sensing and supervising the dot imprints on the paper.
Representative prior art in the field of sensing printed indicia includes German Pat. Specification No. 2,320,962, granted to A. S. Baran et al. on Mar. 17, 1977, which discloses a printing device with an optical sensing head on the printer carriage and which reads the bars on a special program sheet.
German Specification No. 2,617,902, to J. Gross and opened to the public on Nov. 3, 1977, discloses an error indicating device for a wire printer which includes force sensing means in the plane of the platen in face-to-face relationship with the print head and responsive to impact of the print wires and providing signals for interrupting the printing operation. The force sensors which are located at extreme left and right positions in the platen determine whether all the print wires strike with the proper force.
German Specification No. 2,816,542 to T. Ota, opened to the public on Oct. 26, 1978 and corresponding to U.S. Pat. No. 4,184,781, discloses force sensing elements in the platen of a dot printer wherein the drive mechanism drives each dot printing element when the printing elements come to rest on the opposite side of the sensing elements and force errors of each printing element are determined by decreases in the force sensing output signals generated by the drive of the printing elements. The force sensor in the platen is divided into a plurality of individual force sensing elements associated with each print wire to enable energization of all print wires simultaneously during each test operation.
A bar code reader or visual record printer designated as Class 4504 and made by NCR Corporation has a combined ball print head and a read head or optical sensing element consisting of two glass fibers which are sufficiently broad for detecting a printed bar made up of a plurality of dots. The detection of the printed bar takes place during an individual reading run of the carriage along the line of the printed bars.
U.S. Pat. No. 3,476,311, issued to A. P. Feldman on Nov. 4, 1969, discloses a two-dimensional structure encoding typewriter including horizontal and vertical position sensing apparatus which is responsive to the typing of a symbol for providing output signals indicative of the coordinates of each typed symbol to encoding apparatus which converts the signals into a suitable code for recording.